Re: cat as a builtin command

[Bart Schaefer <schaefer@brasslantern.com>]

On Sep 1,  3:53pm, Han Pingtian wrote:
}
} It should be 
} 
}     exec {file}<&0
} 
} right? But I get error mesage for it:
} 
}     localhost% (){
}     function> local fd
}     function> exec {fd}<&0
}     function> read -E -u $fd
}     function> exec {fd}<&-
}     function> }
}     (anon):2: 0: bad file descriptor
}     (anon):read:3: argument expected: -u
}     (anon):4: failed to close file descriptor 0: bad file descriptor

I'm not able to reproduce this.  Is this in a newly started shell?

    torch% (){           
    local fd  
    function> exec {fd}<&0
    function> read -E -u $fd
    function> exec {fd}<&-
    function> }
    hello
    hello
    torch% 

I *suspect* that what happened is that while you were experimenting, some
previous "exec <&-" has already closed descriptor 0.  Closing stdin is
not fatal to an interactive zsh, it maintains its own descriptor for ZLE
to access /dev/tty.

E.g., if I explicitly do:

torch% exec 0<&-   

Then up-history a couple of times and:

torch% (){         
local fd
exec {fd}<&0
read -E -u $fd
exec {fd}<&-
}
(anon):2: 0: bad file descriptor
(anon):read:3: argument expected: -u
(anon):4: failed to close file descriptor 0: bad file descriptor
torch% 

What's puzzling to me is why line 4 says "0: bad file descriptor" rather
than this:

torch% exec {fd}<&-
zsh: parameter fd does not contain a file descriptor

It appears that only an UNSET parameter name triggers the "does not
contain" error; a set-but-empty parameter is treated as 0 and closes
standard input, which is likely how you got into this situation.